Catalyst layer edge protection for enhanced MEA durability in PEM fuel cells

ABSTRACT

A membrane electrode assembly comprising an ionically conductive member and an electrode, wherein the electrode is a smooth, continuous layer that completely covers and supports the ionically conductive member. The electrode further comprises a central region and a peripheral region, wherein a gradient of electrochemically active material exists between the central region and the peripheral region such that a content of the electrochemically active material is greater in the central region than the peripheral region.

FIELD OF THE INVENTION

[0001] The present invention relates to fuel cells and, moreparticularly, to a membrane electrode assembly for a fuel cell.

BACKGROUND OF THE INVENTION

[0002] Fuel cells have been proposed as a power source for electricvehicles and other applications. One such fuel cell is the PEM (i.e.Proton Exchange Membrane) fuel cell that includes a so-called“membrane-electrode-assembly” (MEA) comprising a thin, solid polymermembrane-electrolyte having a pair of electrodes (i.e., an anode and acathode) on opposite faces of the membrane-electrolyte. The MEA issandwiched between planar gas distribution elements.

[0003] The electrodes are typically of a smaller surface area ascompared to the membrane electrolyte such that edges of the membraneelectrolyte protrude outward from the electrodes. On these edges of themembrane electrolyte, gaskets or seals are disposed to peripherallyframe the electrodes. Due to the limitations of manufacturingtolerances, the seals, MEA, and gas distribution elements are notadequately closely aligned. Thus, there is a need for improvedarrangement of these elements.

SUMMARY OF THE INVENTION

[0004] The present invention provides a membrane electrode assemblycomprising an ionically conductive member and an electrode, wherein theelectrode is a relatively smooth, continuous layer that essentiallycompletely covers and supports the ionically conductive member. Theelectrode includes a central region and a peripheral region, wherein agradient of electrochemically active material exists between the centralregion and the peripheral region, such that a content of theelectrochemically active material is greater in the central region thanthe peripheral region. In one embodiment, the active region comprisespolymeric ionomer and catalyzed carbon particles; and the peripheralregion comprises the polymeric ionomer and uncatalyzed carbon particles.In one preferred embodiment, the respective central and peripheralregions comprise respective materials having at least one property whichis approximately the same, namely, at least one of: similar tensilestrength, similar non-standard modulus, similar elongation to break,similar specific gravity, similar water uptake, and similar linearexpansion. In a preferred method of manufacture, a first ink containingthe active region constituents is deposited on a membrane and then asecond ink containing the peripheral region constituents is depositedbefore the first ink cures or dries. Thus, an intermediate region isformed having a catalyst content between that of the active andperipheral regions, as the inks intermingle or particles migrate therebetween. As can be seen, the electrode layer so formed is continuousover the face of the membrane. Thus, stresses engineered by prior artseals or gaskets are obviated.

[0005] Further areas of applicability of the present invention willbecome apparent from the detailed description provided hereinafter. Itshould be understood that the detailed description and specificexamples, while indicating the preferred embodiment of the invention,are intended for purposes of illustration only and are not intended tolimit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

[0007]FIG. 1 is a cross-sectional view of a membrane electrode assemblyaccording to a first embodiment of the present invention;

[0008]FIG. 2 is a cross-sectional view of a prior art membrane electrodeassembly;

[0009]FIG. 3a is a cross-sectional view of a membrane electrode assemblyaccording to a first and second embodiment of the present inventionwhere the anode and cathode electrodes are disposed on an ionicallyconductive member;

[0010]FIG. 3b is a cross-sectional view of a membrane electrode assemblyaccording to a first and second embodiment of the present inventionwhere the anode and cathode electrodes are disposed on gas diffusionmedia;

[0011]FIG. 4 is an exploded, perspective view of a membrane electrodeassembly according to a first and second embodiment of the presentinvention;

[0012]FIG. 5a is a cross-sectional view of a membrane electrode assemblyaccording to a third embodiment of the present invention where the anodeand cathode electrodes are disposed on an ionically conductive member;

[0013]FIG. 5b is a cross-sectional view of a membrane electrode assemblyaccording to a third embodiment of the present invention where a resinmay also be used to coat edges of the diffusion media; and

[0014]FIG. 6 is a graph of average cell potential versus runtimecomparing a prior art MEA with an MEA according to the first embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

[0016]FIG. 1 is a cross-sectional view of a membrane electrode assembly(MEA) according to the present invention. As shown in FIG. 1, the MEA 2includes an ionically conductive member 4 sandwiched by an anodeelectrode 6 and a cathode electrode 8. The MEA 2 is further sandwichedby a pair of electrically conductive members 10 and 12, or gas diffusionmedia 10 and 12. The gas diffusion media 10 and 12 are peripherallysurrounded by frame-shaped gaskets 14 and 16. Gaskets 14 and 16 anddiffusion media 10 and 12 may or may not be laminated to the ionicallyconductive member 4 and/or the electrodes 6 and 8.

[0017] The ionically conductive member 4 is preferably a solid polymermembrane electrolyte, and preferably a PEM. Member 4 is also referred toherein as a membrane. Preferably, the ionically conductive member 4 hasa thickness in the range of about 10 μm-100 μm, and most preferably athickness of about 25 μm. Polymers suitable for such membraneelectrolytes are well known in the art and are described in U.S. Pat.Nos. 5,272,017 and 3,134,697 and elsewhere in the patent and non-patentliterature. It should be noted, however, that the composition of theionically conductive member 4 may comprise any of the proton conductivepolymers conventionally used in the art. Preferably, perfluorinatedsulfonic acid polymers such as NAFION® are used. Furthermore, thepolymer may be the sole constituent of the membrane, containmechanically supporting fibrils of another material, or be interspersedwith particles (e.g., with silica, zeolites, or other similarparticles). Alternatively, the polymer or ionomer may be carried in thepores of another material.

[0018] In the fuel cell of the present invention, the ionicallyconductive member 4 is a cation permeable, proton conductive membrane,having H⁺ ions as the mobile ion; the fuel gas is hydrogen (orreformate) and the oxidant is oxygen or air. The overall cell reactionis the oxidation of hydrogen to water and the respective reactions atthe anode and cathode are H₂=2H⁺+2e⁻ (anode) and ½ O₂+2H⁺+2e⁻=H₂O(cathode).

[0019] The composition of the anode electrode 6 and cathode electrode 8preferably comprises electrochemically active material dispersed in apolymer binder which, like the ionically conductive member 4, is aproton conductive material such as NAFION®. The electrochemically activematerial preferably comprises catalyst-coated carbon or graphiteparticles. The anode electrode 6 and cathode electrode 8 will preferablyinclude platinum-ruthenium, platinum, or otherPt/transition-metal-alloys as the catalyst. Although the anode 6 andcathode 8 in the figures are shown to be equal in size, it should benoted that it is not out of the scope of the invention for the anode 6and cathode 8 to be of different size (i.e., the cathode larger than theanode or vice versa). A preferred thickness of the anode and cathode isin the range of about 2-30 μm, and most preferably about 10 μm.

[0020] The gas diffusion media 10 and 12 and gaskets 14 and 16 may beany gas diffusion media or gasket known in the art. Preferably, the gasdiffusion media 10 and 12 are carbon papers, carbon cloths, or carbonfoams with a thickness of in the range of about 50-300 μm. The gaskets14 and 16 are typically elastomeric in nature but may also comprisematerials such as polyester and PTFE. However, the gaskets 14 and 16 maybe any material sufficient for sealing the membrane electrode assembly2. A preferred thickness of the gaskets 14 and 16 is approximately{fraction (1/2)} the thickness of the gas diffusion media 10 and 12 toabout 1½ times the thickness of the gas diffusion media 10 and 12.

[0021] In accordance with a first embodiment of the invention shown inFIG. 1, the anode electrode 6 and cathode electrode 8 are disposed onopposing surfaces of the ionically conductive member 4 so as tocompletely cover the ionically conductive member 4. Disposing theelectrodes 6 and 8 to completely cover the ionically conductive member 4provides protection for the ionically conductive member 4 from punctureby the fibers of the porous gas diffusion media 10 and 12. Moreover,disposing the electrodes 6 and 8 to completely cover the ionicallyconductive member 4 provides mechanical support throughout the entiresurface of the ionically conductive member 4.

[0022] Before further describing the invention, it is useful tounderstand the problem herein identified in the prior art design.

[0023] The prior art MEA 24, as can be seen in FIG. 2, includeselectrodes 26 and 28 with a much smaller surface area in comparison tothe membrane electrolyte 30 such that edges 32 of the membraneelectrolyte 30 protrude outward from the electrodes 26 and 28. On theseedges 32 of the membrane electrolyte 30, rest sub-gaskets 34 and 36,that are disposed to surround the electrodes 26 and 28. Gas diffusionmedia 38 and 40 sit upon the sub-gaskets 34 and 36. Gaskets 42 and 44surround the gas diffusion media 38 and 40.

[0024] Due to difficulty in manufacturing to tight tolerances, there isa gap 50 between the electrode 26 and sub-gasket 34. A similar gap 52exists between electrode 28 and sub-gasket 36. Such gaps 50, 52 act as aliving hinge, permitting membrane 32 to flex. Such hinge action leads tostress and tears, rips or holes. This also leads to stress as thecompressive force acting on membrane 32 differs due to such differencein height. For example, if the sub-gasket 34 or 36 is higher than theelectrode 26 or 28, the compressive forces on the subgasket 34 or 36will be too high, if the sub-gasket 34 or 36 is shorter than theelectrode 26 or 28, the compressive forces on the electrode 26 or 28will be too high. Thus, the arrangement typical in the prior art causesa small gap formed between the sub-gaskets 34 and 36 and the electrodes26 and 28. This small gap leaves a small portion of the membraneelectrolyte 32 unsupported. Furthermore, if the sub-gaskets 34 and 36are thicker than the electrodes 26 and 28, they form a “step” upon whichgas diffusion media 38 and 40, which are typically porousgraphite/carbon paper, rest. Gas diffusion media 38 and 40 assist indispersing reactant gases H₂ and O₂ over the electrodes 26 and 28 andconduct current from the electrodes 26 and 28 to lands of theelectrically conductive bipolar plates (not shown). As such, in order tofacilitate electrical conductivity between the gas diffusion media 38and 40 and electrodes 26 and 28, the membrane electrode assembly 24needs to be compressed at a high pressure. This puts a great deal ofstress on the unsupported portion of the membrane electrolyte 32 whichmay cause it to develop small pinholes or tears. The pinholes are alsocaused by the carbon or graphite fibers of the diffusion media 38 and 40puncturing the membrane electrolyte 32. These pinholes and tears causethe fuel cell to short and produce a lower cell potential.

[0025] It should be noted that although sub-gaskets 34 and 36 aredepicted in FIG. 2 beneath gaskets 42 and 44, sub-gaskets 34 and 36 arenot necessarily used. More particularly, only gaskets 42 and 44 may bepresent, directly disposed on the membrane electrolyte 30. Nevertheless,due to manufacturing tolerances, gaps 50 and 52 would still be presentand therefore, the membrane electrolyte 30 is still unsupported andsubjected to undue stress when compressed in a stack.

[0026] Accordingly, uniform mechanical support of the ionicallyconductive member 4, which is a very delicate material, provided by thepresent invention is a significant improvement that reduces anypotential variations in compressive force on the ionically conductivemember 4, thereby reducing the possibility of creep and rupture.Moreover, when a perfluorinated sulfonic acid polymer such as NAFION® isused, linear expansion of the ionically conductive member 4 becomes anissue. More particularly, in the presence of water, perfluorinatedsulfonic acid polymers such as NAFION® may have a water intake of up to50% and a linear expansion that ranges between 15 and 50% (15% if themember 4 is unrestrained, and up to 50% if the member 4 is restrained tomovements in only one dimension (the latter is the case for MEAsassembled in a fuel cell stack)). As the overall reaction of the fuelcell produces water as a product, this “swelling” of the ionicallyconductive member 4 may cause the ionically conductive member 4 to beunsupported around its edges.

[0027] When the anode electrode 6 and cathode electrode 8 are disposedto completely cover the entire surface of the ionically conductivemember 4 (see FIG. 1), however, this linear expansion is restricted, andtherefore, the ionically conductive member 4 remains supportedthroughout its entire surface. As stated above, the anode electrode 6and cathode electrode 8 are comprised of catalyst-coated carbon orgraphite particles embedded in a polymer binder which, like theionically conductive member 4, is a proton conductive material such asNAFION®. Although a polymer or ionomer binder such as NAFION® is used,the swelling of the binder does not lead to significant dimensionalchanges of the electrodes since the ionomer in the electrode can expandinto the voids in the electrode (typical void volume fraction ofelectrodes is 50±25%).

[0028] The carbon or graphite particles do not swell in the presence ofwater, are very porous, and have a high surface area. As the anode 6 andcathode 8 become hydrated in the humid fuel cell, the binder swells andfills the pores between the carbon or graphite. Since the swellingbinder fills the pores of the carbon or graphite particles, the linearexpansion of the electrodes 6 and 8 is restricted. As such, when theanode electrode 6 and cathode electrode 8 are adhered to the entiresurface of the ionically conductive member 4, the linear in-planeexpansion of the ionically conductive member 4 is also restricted.

[0029] Furthermore, it should be understood that the anode electrode 6and cathode electrode 8 are disposed over the ionically conductivemember 4 as continuous, smooth layers which provides an essentially flatsurface for the other elements of the MEA 2 to rest upon. This isbeneficial in that when elements such as the diffusion media 10 and 12and gaskets 14 and 16 are compressed along with the MEA 2 (see FIG. 1)in a fuel cell to facilitate and enhance the electrical conductivity ofthe electrons produced in the electrochemical reaction of the fuel cell,the ionically conductive member 4 will be subjected to uniform pressurethroughout its surface. When the ionically conductive member 4 issubjected to uniform pressures throughout its surface, undue stress onthe ionically conductive member 4 will be eliminated. As such, the tearsand pinholes that may develop and shorten the life of the MEA andinhibit the overall cell potential will also be eliminated.

[0030] Although catalyzed carbon or graphite particles dispersed in aproton conductive binder such as NAFION® has been described and ispreferable, the essential aspect of the present invention is thesubjecting of the ionically conductive member 4 to the same mechanicalproperties throughout its entire surface. As such, it is not out of thescope of the present invention to substitute different materials inplace of the carbon or graphite-supported catalyst particles and theproton conductive binder. For example, electrically conductive oxides,and particularly electrically conductive metal oxides may be used assupport material for the catalytically active phase (e.g., Pt,Pt-metals, Pt-alloys, etc.) rather than carbon or, alternatively,unsupported catalysts (e.g., Pt black, Pt-metal blacks, etc.) may beused.

[0031] It is also preferable that the above mentioned catalyst supportmaterials have a particle size equal to or less than 15 μm, bechemically stable in the fuel cell environment (i.e., an acidicenvironment, at anodic potentials (0V vs. RHE) in the presence of H₂, atcathodic potentials (1.2V vs. RHE) in the presence of air or O₂, andtraces of fluoride), and have a sufficient thermal conductivity,preferably equal or greater than carbon or graphite particles.

[0032] In a unique variation of the first embodiment, it may bepreferable that the anode electrode 6 and cathode electrode 8 eachcomprise a central region 18 and a peripheral region 20 as can be seenin FIGS. 3a, 3 b, and 4. The central region 18 comprises a firstcatalyst content and the peripheral region 20, which frames the centralregion 18, comprises a second catalyst content, wherein the firstcatalyst content is greater than the second catalyst content.Specifically, it is preferable that the central region 18 of the anode 6and cathode 8 comprise a catalyst loading in the range of about 0.05-0.5mg/cm² of the catalytically active phase (e.g., Pt). It is particularlypreferable that the central region comprise a catalyst loading of about0.2 mg/cm² of the catalytically active phase (e.g., Pt). The peripheralregion 20 preferably comprises a catalyst loading less than the abovedescribed ranges, and more preferably comprises a zero catalyst loading.

[0033] There is no limitation to how the anode electrode 6 and cathodeelectrode 8 are disposed to protect the ionically conductive member 4and subject the member 4 to uniform mechanical properties. In FIG. 3a,the anode electrode 6 and cathode electrode 8 including the centralregion 18 and peripheral region 20 are coated on the ionicallyconductive member 4 to completely cover the ionically conductive member4. The diffusion media 10 and 12 rest upon the anode electrode 6 and thecathode electrode 8. Gaskets 14 and 16 frame the diffusion media 10 and12 and also rest upon anode electrode 6 and cathode electrode 8 to sealthe assembly 2. The gaskets 14 and 16 and diffusion media 10 and 12 mayor may not be laminated to the anode electrode 6 and cathode electrode8.

[0034] In contrast, as shown in FIG. 3b, the anode, electrode 6 andcathode electrode 8 may be coated onto the diffusion media 10 and 12.Gaskets 14 and 16 are now disposed to contact the ionically conductivemember 4. The diffusion media 10 and 12 including the anode 6 andcathode 8 may or may not be laminated to the ionically conductive member4. Furthermore, the gaskets 14 and 16 may or may not be laminated to thediffusion media 10 and 12.

[0035] It should be understood that when the anode electrode 6 andcathode electrode 8 are coated onto the membrane 4, the anode electrode6 and cathode electrode 8 do not necessarily extend to the edges ofmembrane 4 as shown in FIG. 3a. More specifically, the anode electrode 6and cathode electrode 8 may be coated on the membrane 4 similarly to theconfiguration shown in FIG. 3b, where gaskets 14 and 16 are alsodisposed to contact the ionically conductive member 4.

[0036] It should also be understood that a definitive border between thecentral region 18 and peripheral region 20 does not necessarily exist asshown in FIGS. 3a, 3 b, and 4. More particularly, it should beunderstood that essentially a gradient exists between the central region18 and the peripheral region 20 such that the content of catalystgradually moves from a greater content in the central region 18 tolesser content in the peripheral region 20. This gradient will existover the course of, for example, 1 mm.

[0037] It should also be understood that another important aspect of theinvention is to avoid a discontinuity of material in the anode 6electrode and cathode electrode 8. More specifically, the anodeelectrode 6 and cathode electrode 8 should each exist in this variationas a smooth, continuous layer so that the ionically conductive member 4faces an electrode layer having essentially uniform mechanicalproperties throughout its entire surface which will protect theionically conductive member 4 from stress, over-compression, andpuncture. Moreover, MEA processing (such as hot-pressing todecal-transfer electrodes to membrane) to manufacture MEAs by the priorart design could cause weakening of the catalyst edges. This could bedue to the property of the ionomeric materials to flow at hightemperatures (>90° C.) and high compression, exacerbated by the presenceof prominent catalyst edge. Manufacturing an edge-less MEA byintroducing a periphery makes this a lesser problem.

[0038] Employing an electrode configuration wherein the central region18 has a catalyst content greater than a catalyst content of theperipheral region 20 provides an advantage in that the expensivecatalyst, which preferably comprises metal catalysts such as platinum,palladium, titanium, ruthenium, rhodium, tungsten, tin, or molybdenum,will not be used in areas where the (electro-)chemical reaction isinhibited or not desired. Such an area is located at the edges of theelectrically conductive gas diffusion media 10 and 12.

[0039] Another advantage of a design where the catalyst content in theperipheral region 20 is less than the central region 18 is that thegeneration of heat is suppressed. The electrochemical reaction ofhydrogen and oxygen in the fuel cell produces, in addition to water,heat. In a fuel cell, the heat generated by the electrochemical reaction(or by chemical reaction due to either gas permeation through themembrane or gas cross-over through pinholes in the membrane) istransferred away by the porous gas diffusion media 10 and 12. However,in the first embodiment of the present invention, the anode and cathodeelectrodes 6 and 8 extend outward from the gas diffusion media 10 and 12in order to protect the delicate ionically conductive member 4 fromstress and puncture. Although electrochemical reaction rates are largelydiminished in regions outside of the diffusion media 10 and 12 (due topoor electronic in-plane conduction in the electrodes), heat is stillgenerated due to the catalyst still being present and exposed to thegaseous reactants. As the gaseous reactants have access to the catalyst,the electrochemical reaction of the fuel cell still progresses in theperipheral region 20 that produces heat; particularly in the case ofsmall membrane pinholes, permeation of either reactant (H₂ or O₂) willlead to a chemical reaction producing heat. As such, reducing thecatalyst content over a gradient between the central region 18 and theperipheral region 20, preferably down to zero, will reduce and suppressthe amount of heat generated.

[0040] In the variation of the first embodiment comprising the centralregion 18 and the peripheral region 20, it should be noted thatdifferent materials may be used for the central region 18 and theperipheral region 20 as long as the mechanical properties of each regionare essentially the same so that a discontinuity in properties is notexperienced along the surfaces of the ionically conductive member 4.

[0041] Still referring to FIGS. 3a, 3 b, and 4, a second embodiment ofthe present invention will now be described. As best seen in FIG. 3, andheretofore described above in the first embodiment, the anode electrode6 and cathode electrode 8 comprise a central region 18 and a peripheralregion 20. The central region 18 preferably comprises electrochemicallyactive material, carbon or graphite particles and an ionomer binder. Theperipheral region 20 also contains carbon or graphite particles and anionomer, but in the second embodiment, does not contain anyelectrochemically or chemically active material.

[0042] An electrode configuration wherein the peripheral region 20contains no electrochemically active material further enhances thethermal conductivity characteristics of the peripheral region 20. Asthere is no (electro-)chemically active material present in theperipheral region 20 of the second embodiment, there will be no(electro-)chemical reaction in the peripheral region 20. As such, heatwill not be generated in the peripheral region 20. The peripheral region20 will, however, effectively enhance the conduction of heat away fromthe central region 18 of the electrodes 6 and 8 which will enhance theoperation of the fuel cell. In addition, the peripheral region alsoprovides protection of the membrane 4 from mechanical puncture by thediffusion media 10 and 12.

[0043] For example, the central region 18 and peripheral region 20 (seeFIGS. 3 and 5) or 22 (see FIG. 5) may comprise different particles anddifferent binders. More particularly, particulate matter such as siliconcarbide, titanium dioxide, any other ceramics, or any other materialthat has a sufficient thermal conductivity, preferably equal to orgreater than carbon, may be used in place of carbon or graphiteparticles in the periperal region 20 or 22. These particles may or maynot be electrically conductive. It is also preferable that thisparticulate matter have a particle size equal to or less than 15 μm, bechemically stable in the fuel cell environment (i.e., an acidicenvironment, at anodic potentials (0V vs. RHE) in the presence of H₂, atcathodic potentials (1.2V vs. RHE) in the presence of air or O₂, andtraces of fluoride), and have a sufficient thermal conductivity,preferably equal or greater than carbon or graphite particles

[0044] An example of a binder that may be used in the periperal region20 (see FIGS. 3 and 5) or region 22 (see FIG. 5) in place of the protonconductive binder is polybenzimidazole (PBI). Other binders may besuitable as long as they maintain good adhesion with the ionomericmembrane, are chemically stable in the fuel cell environment (i.e., anacidic environment, at anodic potentials (0V vs. RHE) in the presence ofH₂, at cathodic potentials (1.2V vs. RHE) in the presence of air or O₂,and traces of fluoride), thermally stable up to 150° C., and preferablyup to 200° C., are preferably castable from solutions, and maintain goodretention of their mechanical properties after the casted films enduretemperature excursions up to 150° C. For example, a polymer binder suchas PBI, Kynar, polyester, polyethylene, or any other polymer bindersuitable for a fuel cell environment may be used unitarily.

[0045] As was the case in the first embodiment, different materials maybe used for the central region 18 and the peripheral region 20 besidescarbon or graphite as long as the mechanical properties of each regionare essentially the same so that a discontinuity in properties is notexperienced along the surfaces of the ionically conductive member 4

[0046] Now referring to FIGS. 5a and 5 b, a third embodiment of thepresent invention will be described. As in the first and secondembodiments, the MEA of the third embodiment preferably comprises acentral region 18 of electrochemically active material, carbon orgraphite particles, and an ionomer. However, other catalytically activematerials as described in the first embodiment may be used in region 18.In the peripheral region 20 either uncatalyzed support materials of thesame nature as in region 18 may be used or other particles and bindersas described in the second embodiment can be used. The MEA of the thirdembodiment, however, also comprises a sealing region 22 adjacent theperipheral region 20 (shown in FIG. 5a). The sealing region 22 preventsthe leakage of the gaseous reactants from the fuel cell, and iscomprised of a resin such as polyvinylidene fluoride dispersed into partof the peripheral region 20. Polyvinylidene fluoride is a thermoplasticresin sold under the tradename Kynar® by Elf Atochem.

[0047] Although the addition of the sealing region 22 prevents theleakage of the gaseous reactants from the fuel cell, the sealing region22 also provides the benefit of enhancing the mechanical strength andtoughness of the edges of the MEA assembly such that the ionicallyconductive member 4 will be further protected from stress and puncturefrom the gas diffusion media 10 and 12 when compressed in a fuel cellstack. The anode electrode 6 and cathode electrode 8 each compriseporous carbon or graphite particles as well as ionimeric binder and arecharacterized by a large void volume fraction (50±25%) which may enablethe escape of the reactant gases. The addition of the sealing region 22,comprising the sealing material such as Kynar®, fills these remainingvoid areas to provide an increased mechanical strength in addition tomore effectively sealing the MEA 2 from the lateral escape of thereactant gases.

[0048] It should be understood that although Kynar® is preferred, anyresin may be used as long as it has a low permeability to gases andliquids and is resistant to most chemicals and solvents. Furthermore,any resin one may choose should be heat resistant to temperaturesgreater than 150° C., and more preferably greater than 200° C., so thatit may withstand the harsh fuel cell environment. An example of such aresin that may be used, but should not be limited to in substitution forKynar®, is an epoxy resin.

[0049] Furthermore, as shown in FIG. 5b, the resin may also be used tocoat edges of the diffusion media 10 and 12 to form a seal 54. Coatingthe edges of the diffusion media with the seal 54 can thereforeeliminate the use of gaskets 14 and 16. It should be noted, however,that gaskets 14 and 16 may still be utilized in FIG. 5b (althoughgaskets 14 and 16 are not shown) if desired.

[0050] In each of the above embodiments, the central region 18 andperipheral region 20 may be catalyzed with finely divided catalyticparticles so that the weight ratio of catalytic particles to carbon orgraphite particles of the peripheral region 20 is less than that of thecentral region 18. It is evident that where the peripheral region 20does not contain any catalyst particles and the central region 18 iscatalyzed, this condition will be met. In the embodiment where catalyticparticles are included in both regions, it is preferable that the weightratio of catalytic particles to carbon particles in the central region18 is greater than that of the peripheral region 20.

[0051] A method of preparing a MEA 2 according to the present inventionwill now be described. In order to prepare the anode 6 and cathode 8 ofthe MEA, catalyzed carbon particles are prepared and then combined withthe ionomer binder in solution with a casting solvent. Preferably, theanode 6 and cathode 8 comprise ⅓ carbon or graphite, ⅓ ionomer, and ⅓catalyst. Preferable casting solvents are aqueous or alcoholic innature, but solvents such as dimethylacetic acid (DMAc) ortrifluoroacetic acid (TFA) also may be used.

[0052] The casting solution is applied to a sheet suitable for use in adecal method, preferably the sheet is a Teflonated sheet. The sheet issubsequently hot-pressed to an ionically conductive member 4 such as aPEM. The sheet is then peeled from the ionically conductive member 4 andthe catalyst coated carbon or graphite remains embedded as a continuouselectrode 6 or 8 to completely form the MEA 2.

[0053] In order to prepare electrodes 6 and 8 that comprise a centralregion 18 and a peripheral region 20, two casting solutions may beemployed. More particularly, a first casting solution is applied to thesheet suitable for a decal method to form the central region 18 of theelectrode 6 or 8. The first casting solution has a predetermined contentof catalytic particles contained therein. A second casting solution isthen applied to the sheet to peripherally frame the central region 18 asa peripheral region 20. The second casting solution also has apredetermined content of catalytic particles. In accordance with thepresent invention, the second casting solution has content of catalyticparticles less than the first casting solution, or it may contain nocatalyst at all. The sheet is then subsequently hot-pressed to aionically conductive member 4 such as a PEM and then peeled from theionically conductive member 4 and the central region 18 and peripheralregions 20 remain embedded to completely form the MEA 2. In an alternateembodiment, the electrode is formed on the membrane or on a layer ofdiffusion media.

[0054] The second casting solution is applied directly after the firstcasting solution has been applied such that the first casting solutionhas not completely dried or solidified. Applying the casting solutionsin such a manner will ensure that smooth, continuous electrodes 6 and 8will be formed on the ionically conductive member 4 so that there is nodiscontinuity in the electrodes 6 and 8. Furthermore, applying thecasting solutions in such a manner will allow the gradient to formbetween the central region 18 and the peripheral region 20 of theelectrode 6 or 8. In a variation of the above method, it may bepreferable to apply the first and second casting solutions essentiallysimultaneously.

[0055] With respect to a method of preparing an MEA according to thethird embodiment including the sealing region 22, the present inventionshould not be limited to a particular method of applying the sealingregion 22. For example, the sealing region 22 may be painted or sprayedonto the peripheral region 20 and allowed to fill the remaining voidregions. Also, the sealing material 22 may be included in the secondcasting solution.

[0056] A durability experiment comparing an MEA according to the firstembodiment of the present invention (FIG. 1) with a prior art MEA (FIG.2) will now be described. The durability experiment measured the averagecell potential over time for both the MEAs according to the firstembodiment of the present invention and the prior art MEA.

[0057] The durability testing was conducted at high-temperatureaccelerated conditions (RH_(anode/cathode): 75/50, 200 kpa(g),T_(stack): 95° C.). Each MEA utilized a PEM made from a 25 μm thickmembrane with 1100 EW (equivalent weight) membrane (extruded Nafion 111in the sulfonylfluoride form was purchased from DuPont and ion-exchangedby IonPower, Inc.). These membranes (further referred to as N111) werechosen based on data from prior testing that indicated the N111membranes were the weakest in terms of durability. As such, ifdurability were to be achieved by the improved construction of thepresent invention with a weaker membrane, a more robust membrane wouldexhibit an even greater likelihood of increased durability.

[0058] Now referring to FIG. 6, it can be seen that the prior art MEA 24exhibited a lower open circuit cell potential at approximately 45 hours.The drop in the cell potential for the prior art MEA 24 can beattributed to ohmic shorts developing in the MEA 24. These shortsdevelop when the membrane 30 is punctured by the porous fibers of thegas diffusion media 38 and 40 and by the membrane 30 being stressed dueto compression of the MEA 24. At later time (ca. 80 hours), pinholes inthe membrane 30 develop, worsening over time and leading to MEA failure.

[0059] Comparing open-circuit potentials (OCV) with equal reactantpressures against OCV with anode (H₂) pressure higher than cathode (air)provides a valuable insight into pinhole formation. If the OCV with apressure differential dropped in comparison to OCV with equal pressures,it indicates that there is significant H₂ crossover, i.e pinholeformation. As can be seen in FIG. 6, severe pinholes develop in theprior art membrane 30 at approximately 80-90 hours. In contrast, the MEA2 according to the first embodiment of the present invention exhibits ahealthy cell potential up to 175 hours, whereupon gasket failure(puncture of the membrane at the gasket ege) occurred which would notoccur in an optimized fuel cell hardware.

[0060] Upon completion of the experiment, both the prior art MEA 24 andthe MEA 2 according to the first embodiment of the present inventionwere disassembled. The prior art MEA 24 had severe pinhole damage at theelectrode-membrane and diffusion media-membrane edges. The MEA 2 of thefirst embodiment of the present invention, wherein the electrodes 6 and8 are extended to cover and support the membrane 4, had no damageevident. Each failure in MEA 2 was only at the gasket edge and would notoccur in optimized fuel cell hardware. As such, it is evident that theimproved MEA construction of the present invention provides enhancedprotection and support of the membrane.

[0061] As can be seen, the invention provides a membrane electrodeassembly wherein the electrode has a peripheral extent preferably atleast as great as the membrane, and preferably is essentiallycontinuous, so as to avoid difficulties with the hinge effect anddifficulties with stepwise differences in height, as described hereinwith regard to FIG. 2. Preferably the electrode of the present inventionhas a major surface which is at least as great or essentiallycoextensive with the major surface of the membrane which it supports. Inthis arrangement, the electrode functions to minimize flexing of themembrane by avoiding the hinge effect heretofore present on the basis ofdiscontinuity between layers and height differences between layers whichwould lead to non-uniform compression and compressive creep of themembrane.

[0062] The electrode of the invention is conveniently cast byconventional means to form an electrode film. In the electrochemicalactive regions of the electrode film, catalyst particles catalyzeelectrochemical reaction between fuel and an oxidant. In peripheralregions of the film adjacent the active area, the catalyst content isless or such peripheral areas are essentially devoid of catalyst. Theterm catalyst content refers to less catalyst per unit weight of castfilm area or less weight ratio of catalyst particles to carbonparticles. In either case, the catalyst loading in the active area isrelatively high and the catalyst loading in the peripheral, non-activearea, is low or essentially zero. Preferably, where the continuous filmis formed by application of a first casting solvent, which contains ahigh catalyst loading, and a second casting solvent which does notcontain catalyst, and where the second casting solution is appliedbefore the first casting solution has completely dried to form a film orhas not yet cured. Such continuous film of essentially equivalent heightthroughout is formed and having a gradient of catalyst loading from theactive area through the peripheral area. Further, depending on thedegree to which the first casting solution has cured before the secondcasting solution is applied, an interface area may be formed between theactive region and the peripheral region, having an intermediate catalystcontent which essentially drops to zero at the far edge of theperipheral area.

[0063] The description of the invention is merely exemplary in natureand, thus, variations that do not depart from the gist of the inventionare intended to be within the scope of the invention. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention.

What is claimed is:
 1. An assembly for a fuel cell comprising: anionically conductive membrane having a major surface; an electrodeadjacent said major surface and having a peripheral extent essentiallythe same as said major surface, said electrode, including a centralregion, and a peripheral region; said central region comprising a binderand a first group of particles and said peripheral region comprising abinder and a second group of particles; and wherein said first group ofparticles is catalyzed, and said second group of particles isuncatalyzed or has a catalyst content less than the catalyst content ofsaid first group of particles.
 2. The assembly according to claim 1,wherein said catalyzed particles are catalyzed carbon particles, andsaid binder of said central and peripheral regions is the same.
 3. Theassembly according to claim 2, wherein said carbon particles have aparticle size equal to or less than 15 μm.
 4. The assembly according toclaim 2, wherein said catalyst is selected from the group consisting ofplatinum, palladium, titanium, ruthenium, rhodium, tungsten, tin,molybdenum, and alloys thereof.
 5. The assembly according to claim 1,wherein said central region and said peripheral region each providesubstantially similar mechanical support to the ionically conductivemember, thereby minimizing flexing of said membrane.
 6. The assemblyaccording to claim 1, which further comprises an electrically conductivemember adjacent to said electrode, said electrically conductive memberis a gas diffusion medium selected from the group consisting of carboncloth, carbon foam, and carbon paper.
 7. The assembly according to claim1, wherein said peripheral region is thermally conductive.
 8. Theassembly according to claim 1, wherein said electrode is in the form ofa continuous layer, and said peripheral region encompasses said centralregion.
 9. The assembly according to claim 5, wherein said peripheralregion minimizes contact between said electrically conductive member andsaid membrane.
 10. The assembly according to claim 1, wherein saidperipheral region is electrically conductive.
 11. The assembly accordingto claim 2, wherein said membrane comprises a polymeric ionomer which isthe same as said binder of said central and peripheral region.
 12. Amembrane electrode assembly comprising: an ionically conductive membranehaving a major surface; an electrode at said major surface, saidelectrode defining a continuous layer supporting said membrane andcomprising a central region and a peripheral region; and wherein agradient of electrochemically active material exists between saidcentral region and said peripheral region such that a content of saidelectrochemically active material is greater in said central region thansaid peripheral region.
 13. The membrane electrode assembly according toclaim 12, wherein said electrode restricts movement of said membrane.14. The membrane electrode assembly according to claim 12, wherein saidelectrode layer further comprises a sealing region outboard of saidperipheral region, wherein said central, peripheral and sealing regionscollectively form said continuous layer and provide continuous,consistent mechanical support to the membrane, thereby minimizingflexing of said membrane.
 15. The membrane electrode assembly accordingto claim 12, which further comprises an electrically conductive memberadjacent said electrode, and wherein said electrode layer maintains saidelectrically conductive member spaced from said ionically conductivemember.
 16. An assembly for a fuel cell comprising: a proton exchangemembrane with a first surface and a second surface; an anode electrodedisposed at said first surface; a cathode electrode opposing said anode,and disposed at said second surface; gas diffusion media disposed atsaid anode and cathode, said anode electrode and said cathode electrodeeach comprising a central region and a peripheral region; said centralregion including an electrochemically active material; and saidperipheral region including a thermally conductive material and asealing material; and wherein said anode and said cathode are each inthe form of continuous layers that support said proton exchangemembrane, minimize flexing of said membrane, and prevent said gasdiffusion media from contacting said proton exchange membrane.
 17. Theassembly according to claim 17, wherein said electrochemically activematerial comprises a plurality of catalyst-coated particles dispersed inan ionomer.
 18. The assembly according to claim 17, wherein saidthermally conductive material comprises a plurality of particlesdispersed in an ionomer.
 19. The assembly according to claim 20, whereinsaid thermally conductive material is also electrically conductive. 20.The assembly according to claim 17, wherein said sealing material is athermoplastic resin.
 21. The assembly according to claim 1, wherein saidthermally conductive material is adjacent to said central region andsaid sealing material is outboard of said thermally conductive material.22. A method for manufacturing an electrode for a membrane electrodeassembly, comprising: applying a first casting solution to a firstregion on a surface of a substrate, said first casting solutioncomprising carbon particles and an ionomer; applying a second castingsolution to a second region of said surface of said substrate, saidsecond casting solution comprising carbon particles and an ionomer;wherein said first region is a centrally located region on the surfaceof said substrate, and said second region is outboard of said centrallylocated region on the surface of said substrate; wherein said firstsolution comprises catalyst and said second solution has less catalystcontent than said first solution; and drying each of said appliedsolutions to form a continuous electrode film.
 23. The method of claim24 wherein said second casting solution is applied before said firstcasting solution has dried to a film.
 24. The method of claim 24 whereinsaid second solution does not contain catalyst.
 25. The method of claim24 wherein said substrate is an ionically conductive membrane.
 26. Themethod of claim 24 wherein said continuous electrode film is removedfrom said substrate and then pressed to an ionically conductivemembrane, thereby forming a membrane electrode assembly.